Pipes insulated with rigid polyurethane foam are principally used in providing heating services. The composite plastic-jacketed pipe system is universally used for such purposes. This system comprises factory-insulated pipes consisting of a steel heating medium pipe, a polyethylene jacketing pipe together with rigid polyurethane foam as the insulating material. The pipes are laid in a sand bed. The composite plastic-jacketed pipe heating line comprises a composite of these three components.
Insulation is customarily achieved by filling the cavity between the steel pipe and the heating medium pipe with foam.
The preinsulated pipes are suitable for direct laying in the ground in a sand bed. Space requirements are low. The plastic-jacketed pipe may be adapted to construction circumstances and welded together at any desired point. Considerable cost savings are achieved due to the composite system of the plastic-jacketed pipe.
Other techniques, such as the shrouded conduit system require considerably costlier laying techniques.
The rigid polyurethane foams which are customarily used today are designed for continuous operating temperatures of up to 130.degree. C. with short peaks of up to 140.degree. C. This is adequate for most Western European district heating networks. Eastern European power stations, however, supply substantially higher flow temperatures, which may reach 200.degree. C. The rigid polyurethane foam which is currently customarily used is not suitable for such temperature ranges. As a solution, systems with a two-layered structure for the thermal insulation are known. The inner layer exposed to high temperatures consists of inorganic mineral fibers. Rigid polyurethane foam is used as a second layer of insulation. However, this technique no longer provides a composite system as the adhesion of the mineral fibers on the steel heating medium pipe to the polyurethane foam is very small. Such a "sliding system" requires considerably costlier compensation for axial displacement forces and the installation of fixed points.
The object of the present invention was to provide thermal insulation suitable for high temperature service which forms a composite system with the steel and jacketing pipe.